1. Field of the Invention
The subject invention relates to fuel metering systems, and more particularly, to an improved cam seal for a variable displacement fluid pressure vane pump for use with gas turbines.
2. Background of the Related Art
For many years, main engine fuel pumps have been fixed displacement gear pumps. Although such pumps are durable they are also inefficient. Fixed displacement vane pumps were developed in order to overcome certain deficiencies of gear pumps. An example of such a pump is disclosed in U.S. Pat. No. 4,354,809, the disclosure of which is herein incorporated by reference in its entirety.
Vane pumps include a rotor element that has slots for supporting radially movable vane elements. The rotor element is mounted within a cam member defining a cam surface. The cam surface has a fluid inlet port through which fluid is delivered to the low pressure inlet areas of the rotor surface. The fluid is subsequently compressed and discharged from the high pressure outlet areas of the rotor surface as pressurized fluid.
Variable displacement vane pumps are known, as disclosed for example in U.S. Pat. Nos. 5,545,014 and 5,545,018, the disclosures of which are herein incorporated by reference in their entireties. These pumps contain a swing cam element which pivots relative to the rotor element, so as to change the relative volumes of the inlet and outlet discharge areas and thereby vary the displacement capacity of the pump.
Variable displacement vane pumps often have leakage problems in the high pressure discharge arc area. Spring biased cam seal elements that frictionally engage the faces of the swing cam in the discharge arc area have been designed to overcome these problems, as described for example in U.S. Pat. No. 5,783,500, the disclosure of which is incorporated by reference herein in its entirety.
In the past, seal elements associated with the swing cam have been aligned with the horizontal centerline of the pump. Thus, the cam seals follow the cam stroke. If one or both of the cam seals should happen to bind up due to contamination and be unable to follow the cam, a major leak path would result. In the worst case, the sealed high pressure cavity on the outer diameter of the cam will become inlet pressure, and internal leakage will increase across the fixed clearance between the cam and the sideplates. It is readily apparent that a solution to this problem is necessary.
The subject invention is directed to an improved cam seal arrangement for a variable displacement vane pump which solves the problems associated with cam seals on prior art vane pumps. In particular, the subject invention is directed to a variable displacement vane pump in which the cam seals are located along the arc defined by the cam as it swings about its pivot point relative to the rotor member. Consequently, the seals function as static seals, and will not bind up in their slots. This will minimize cam seal leakage if a seal fails to follow the cam stroke.
In accordance with a preferred embodiment of the subject invention the variable displacement vane pump disclosed herein includes a pump housing having a cylindrical interior chamber defining a central axis through which a vertical centerline and a horizontal centerline extends. A cylindrical rotor member is mounted for rotational movement within the interior chamber of the pump housing about an axis aligned with the central axis of the interior chamber. The rotor member has a central vane section including a plurality of circumferentially spaced apart radial vane slots formed therein. Each vane slot supports a corresponding vane element which is mounted for radial movement therein, and each vane element has an outer tip surface.
A cam member is mounted for pivotal movement within the interior chamber of the pump housing about a fulcrum aligned with the vertical centerline of the interior chamber. The cam member defines a cam body having a circular bore extending therethrough for receiving the rotor member. The circular bore forms a cam chamber defining a smooth cam surface making continuous contact with the outer tip surfaces of the vane elements during the rotation of the rotor member. The cam body has opposed lateral sealing lands formed thereon which have arcuate sealing surfaces that define segments of a cam arc through which the cam member pivots or swings relative to the rotor member.
The variable displacement vane pump of the subject invention further includes lateral cam seals supported within the interior chamber of the pump housing for sealingly isolating the high pressure zone of the pump from the relatively lower inlet pressure of the pump. Each cam seal is biased into a continuous contact position with an adjacent sealing surface of the cam member. The cam seals are oriented on each end of a chord of the cam arc. The chord of the cam arc extends parallel to and is located below the horizontal centerline of the interior chamber. Consequently, the cam seals act as static seals which are less likely to bind up during operation and cause internal leakage across the fixed clearance between the cam member and the sideplates.
Preferably, the variable displacement vane pump of the subject invention includes opposed sideplates disposed within the interior chamber of the pump housing. The sideplates support the rotor member and cam member therebetween. An axial spacer having an axial thickness slightly greater than an axial thickness of the cam member is preferably positioned between the opposed sideplates for reducing or eliminating friction between the sideplates and the cam member. The static cam seals are supported by the axial spacer and are oriented on each end of a chord of the cam arc through which the cam member swings so as to prevent fuel leakage between the high and low pressure zones formed in the area defined between the sideplates.
These and other unique features of the subject invention will become more readily apparent from the following description of the drawings taken in conjunction with the description of the preferred embodiment.